The branded thetford stuff is the only stuff that stops any rancid smell.
We've tried every option, coke, washing power, but the smell can still be awful when empting.
Its not a huge expense, a small amount capful every few tanks, and no smell when emptying. We use less than the recommended dose.
(We have traps so never have internal smells, but I don't like leaving a stink bomb at disposal points - though seems I'm in the minority on this! Mines electric dump too so I don't even need to be near it, but still.....)
+1 for this. Tried the rest and now using the best